Internal combustion engines may use variable cam timing (VCT) to improve fuel economy and emissions performance of a vehicle. The VCT device may include a vane type cam phaser that is controlled by an electromechanically actuated spool valve. The spool valve may direct flow of a hydraulic fluid, such as oil, from one side of the vane to the other, such as from a retard side to an advance side. The VCT device may include more than one oil circuit connecting one side of the vane to the other through which the flow of a hydraulic fluid may be directed. The phaser may be oil pressure actuated, wherein the actuation of the phaser is dependent on oil pressure in the circuit. Alternatively, the phaser may be cam torque actuated wherein the actuation of the phaser is dependent on torque generated during cam actuation.
One example of a cam torque actuated VCT phaser is shown by Smith et al. in U.S. Pat. No. 8,356,583. Therein, the VCT device is configured with a hydraulically activated locking pin in an intermediate position (herein also referred to as a mid-lock position). Conventional VCT devices may include a locking pin at one end of the range of the phaser. The VCT device of Smith also utilizes two independent oil circuits, herein referred to as the phasing circuit and the detent circuit. In the mid-lock VCT phaser of Smith, a piloted valve is included in the phaser's rotor assembly and is moveable from a first position to a second position. When the piloted valve is in the first position, hydraulic fluid is blocked from flowing through the piloted valve. When the piloted valve is in the second position, hydraulic fluid is allowed to flow between a detent line from the advance chamber and a detent line from the retard chamber through the piloted valve and a common line, such that the rotor assembly is moved to and held in the intermediate phase angle position relative to the housing assembly. Detent lines communicating with the advance chamber or retard chamber are blocked when the VCT phaser is at or near the intermediate position. The spool valve has three regions of operation, namely Detent (or Auto-Lock), Retard, and Advance in the specified order. The auto-lock region may hereupon be referred to as the detent region. Specifically, when the spool valve is commanded to the retard or advance regions, the piloted valve is in the first position, and fluid is blocked from flowing through the detent circuit lines. Additionally, fluid may flow from one side of the vane to the other via the phasing circuit lines. When the spool valve is commanded to the detent region, the piloted valve is in the second position, and fluid is free to flow from the advanced or retarded chamber, through the detent lines and the piloted valve, and into the opposite chamber through a common fluid line. Additionally, fluid is blocked from flowing through the phasing circuit lines.
However, the inventors herein have identified potential issues with such a VCT system. If the spool valve is commanded from a low retard region or the advance region to the detent region, it must physically travel through a high retard region. In the instance that a retarded cam torsion is experienced while the spool valve is travelling through the high retard region, the cam phaser may change its position by several degrees in the retarded direction immediately before reaching the detent region and auto-locking. This may increase the time needed for the detent circuit to adjust the cam phaser position to the neutral position. Additionally, this may create delays in subsequent engine commands that require the cam phaser to be in a hard locked position.
In one example, the above issue may be at least partly addressed by a method for an engine, comprising: in response to a desired cam timing at a mid-lock position with a locking pin engaged, moving a spool valve to move a cam timing phaser to a position advanced of the mid-lock position; holding the phaser at the position advanced of the mid-lock position; and then moving the spool valve to a detent region while a cam torsional pulse occurs. In this way, if retarded cam torsions occur during a time when they may actuate a cam phaser position movement, they may be advantageously used to move the cam phaser position toward a neutral position where the locking pin may be engaged.
As an example, the engine controller may request a cam phaser be held in the intermediate position (mid-lock position) with the locking pin engaged at a time when the spool valve is in the advance or retard region. In response to the request, a spool valve command may be adjusted so that the spool valve can move the cam phaser to a position slightly advanced of the mid-lock position. The selected position may be advanced from the mid-lock position by a degree of advance based on an expected magnitude and number of cam torsions. The spool valve may then travel to the detent region of operation, thus activating the detent circuit hydraulic control.
In this way, by pre-positioning a cam phaser at a position advanced of a mid-lock position, even if retarded cam torsions do occur during the movement of the spool valve through the retard region, the retarded cam torsions may move the cam phaser closer to the desired mid-lock position which is required for engaging the locking pin. Even if the cam phaser is not affected by retarded cam torsions, the cam phaser may still be close to the mid-lock position because the predetermined advance phase position may not be large. By reducing the occurrence of unwanted position adjustments arising from spool valve travel through a retard region, the time associated with engaging a locking pin of a VCT phaser may be made more consistent.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.